It has long been a customary practice for subscription television communication systems--a system in which the reception of a television program signal, generally recoverable by a group of television receivers, is limited to specifically authorized receivers--to scramble or encode their television transmission signals to prevent unauthorized reception by individuals who have not paid a subscription fee. Conventional hard-wired community antenna television systems (CATV), for example, are representative of a typical subscription television communication system, although other systems have been developed that employ radio frequency (RF) transmission to subscribers instead of hard-wiring. If the subscription television system provides more than one channel, it is also desirable to provide selected scrambling or securing of individual channels that are used to provide premium services.
One of the methods most commonly employed to prevent unauthorized reception to add a scrambling signal or interfering carrier signal to the television transmission signal between its aural and visual carriers. The scrambling signal is added to the television transmission signal to provide additional information which causes a television receiver to reconstitute the scrambled television transmission signal in an incoherent form. The scrambling signal is removed by passing the scrambled television transmission signal through a tuned notch filter at the site of an authorized television receiver.
Tuned notch filters to be utilized in the video/audio frequency removal or descrambling process must meet several critical requirements. For example, it is critical that the tuned notch filter have a high degree of stability and reliability. In addition, the size of the notch filter must remain small, preferably less than 0.825 inch diameter, to enable the notch filter to be utilized in typical installations where one or more filters are mounted on a directional tap on a strand, in a pedestal, or some other small enclosure. The notch filter must also be capable of a large degree of attenuation at the center frequency of the notch with very sharp skirts and a narrow bandwidth to minimize unwanted attenuation of adjacent channels. As the frequency requirements increase, sharper skirts are required to maintain the desired bandwidth while minimizing interference with adjacent channels.
In order to achieve sharper skirts, the notch filter must have a high Q (Quality Factor), i.e., the bandwidth is minimized by optimization of the Q of the notch filters' inductors and capacitors. In most applications, the Q of the notch filter can be improved by utilizing inductors having larger winding diameters with larger diameter wire. In decoding and descrambling applications, however, the Q cannot be improved by utilizing larger coils due to the above-described size and corresponding layout restrictions.
In view of the above, it is an object of the invention to provide a highly reliable, narrow and stable notch filter having a high Q while minimizing the overall size of the notch filter. Further objects, features and advantages of the invention will become apparent from the ensuing detailed description of the preferred embodiment of the invention taken in connection with the accompanying drawings